Not on Arena (1): Kili the Resourceful. Arena will skip it on import.
Missing sideboard cards? The full 15-card sideboard runs about $33.99.
Buy sideboard on ManapoolGolgari is loaded with discard (Thoughtseize, Duress-adjacent hands) and cheap removal (Fatal Push, Abrupt Decay, Go for the Throat), so Knight of Grace's hexproof from black and extra power dodges a huge swath of their removal suite while pressuring Sheoldred. Nahiri, the Unforgiving gives you card selection with its loot and temporary recursion that doesn't care about their one-for-one removal: its 0 ability exiles a creature or Equipment card from your graveyard (with mana value less than Nahiri's loyalty) and makes a hasty token copy, but that token is exiled at the beginning of the next end step, so it is a one-turn burst rather than a permanent rebuy. Wear // Tear's enchantment half answers Unholy Annex // Ritual Chamber before the Demon token comes down. Skrelv and Super-Soldier Serum are cut since they're low-impact against efficient removal.
Izzet Cauldron leans on cheap removal (Fiery Impulse, Prismari Charm) and countermagic (Stubborn Denial), so Surge of Salvation protects your equipped threats from burn and grants hexproof against their removal-heavy plan for a turn. Wear // Tear's artifact mode can blow up a problematic Reckoner Bankbuster if they bring it in, keeping your clock uncontested. Skrelv is a low-impact 1/1 against a deck with ample cheap removal, and Shadowspear is replaced since Surge already grants temporary protection and the opponent has little lifegain to disrupt. Keep pressuring quickly since their removal is efficient but their clock is slow.
UBR grinds with card advantage engines like Fable of the Mirror-Breaker and The Theorist, Jace Beleren, backed by burn like Fiery Impulse and Volcanic Spite and removal from Indomitable Creativity, so Nahiri, the Unforgiving gives you a planeswalker that can loot with its +1 and, with its 0, temporarily bring back a creature or Equipment card from your graveyard (its mana value must be less than Nahiri's loyalty). The token copy has haste but is exiled at the beginning of the next end step, so it is a burst of value rather than a lasting rebuild. Wear // Tear's enchantment mode can remove Fable's Goblin-making Saga before it flips, or hit other problematic enchantments. Shadowspear and Chainsaw are cut because they are modest, incremental effects against a deck that will out-value you with card draw if the game goes long, and this matchup rewards more impactful plays than incremental pump.